The skin consists of a number of overlapping layers of cells. The outermost layer of the skin is called the stratum corneum and consists of dead keratinized cells. This layer protects the skin from physical and atmospheric harm, acting as a barrier to external dangers. The stratum corneum, compared with the lower layers of the epidermis, is rather dry. Lack of moisture in the lower layers of the skin results in a wrinkled and aged look. Lack of moisture in the strateum corneum, however, is even more noticeable in that this is the layer that we see. The dryness is marked by roughness, increased flakiness, and in more severe cases, cracks and actual peeling. The skin may appear reddened and even inflamed if the dryness is sufficiently acute. It has been shown, by at least one scientist, that the stratum corneum remains soft and pliable only as long as the moisture content exceeds 10%. Below this, the skin becomes hard and brittle and develops an opacity.
In the lower layers of the skin, degenerative changes occur with age whereby not only moisture is lacking, but also a major amount of lipoidal or fatty substances. In cosmetic practice, it is the outermost layer of the skin which can best be benefitted by application of external lotions and creams. If this outermost layer can be made to look plump, transparent and healthy, the overall skin texture will assume a more youthful appearance, however, studies of the epidermis indicate that the stratum corneum is capable of absorbing and retaining only moisture. It will not accept lipid or fatty substances. Such materials used for moisturization of the skin have no direct effect whatsoever in increasing the hydration of the epidermal cells.
As the result of aging, exposure to various climatic conditions, such as, sun and wind and other factors in addition to loss of moisture in the epidermal layers of the skin, it has been found that loss of elasticity and skin tone and texture may occur through degradation of certain complex polypeptides present in the skin such as elastin and collagen, among others.
The collagens, a family of closely related proteins, are the main fibrillar components of the connective tissues and the major extracellular proteins of the human body. Examination of collagen in connective tissues by light microscopy demonstrates that collagen is deposited as large bundles of regularly oriented fibers which on further examination can be shown to be composed of fibrils and microfibrils. The microfibrils are aligned in a parallel manner which results in a pattern of cross-striations or bands which can be visualized by electron microscopy. The most prominent cross-striations appear as repeating bands which are spaced approximately 70 nm apart.
The basic collagen molecule has an approximate molecular weight of 290,000, and it is composed of three polypeptide chains, each having a molecular weight of about 94,000. These three polypeptides, so-called a chain, are coiled on each other much like strands of rope, so that the collagen molecule has a triple-helical structure. This unusual helical conformation gives the molecule a rigid, rodlike shape with approximate dimensions of 1.5.times.300 nm.
Despite the high molecular weight of native collagen and their high degree of insolubility, they may be degraded and solubilized by various procedures including acid and alkali treatment and subjection to enzymatic processes. Such procedures are described in numerous prior art patents and publications, among which for example, attention may be directed to U.S. Pat. Nos. 3,475,404, 3,548,056 and 4,140,537, and the publication of Todd and Biol, "Soluble Collagen--New Protein for Cosmetics" D & CI (Drug and Cosmetic Industry), October, 1975, pp. 50-56 and 134-138.
It has been stated that one of the characteristic features of collagen is that under physiologic conditions collagen molecules spontaneously assemble into insoluble fibers. This observation previously presented a problem in that it was difficult to visualize how a collagen molecule could be synthesized inside the cell and then secreted into the extracellular space without the molecules prematurely assembling into insoluble fibers. This question has now been answered by the demonstration that collagen is initially synthesized as a larger precursor molecule, pro-collagen, which is soluble under physiologic conditions. (Dermatology in General Medicine, Fitzpatrick et al, McGraw - Hill 1979, Chapter 5, page 166).
As referred to in the Todd and Biol publication referred to above, it has been suggested that "a feature of ageing is the progressive insolubilization of collagen through cross-linking and aggregation to produce a form of the protein more resistant to chemical attack, less able to retain moisture and more rigid in structure. The most obvious manifestation of these changes is the development of dry, wrinkled skin that has lost its inherent elasticity. One piece of evidence used to substantiate this theory is the frequent observation that the ratio of soluble to insoluble collagen is greater in young than in older animals--as demonstrated by the greater proportion of the total collagen of skin that is readily extracted."
It has also been suggested that cosmetic application of soluble collagen preparations supplies the skin with soluble collagen and induces formation of new fibrils, thus arresting the loss of natural soluble collagen and compensating for its loss.
The Todd and Biol publication further points out, however, that it is also difficult to visualize how soluble collagen cosmetic treatment could lead to absorption of soluble collagen for fibrillogenesis. "The molecular parameters of the tropocollagen macromolecule in solution have been deduced as about 3000A by 16A with a weight average molecular weight of about 300,000 and this alone must preclude skin penetration and absorption of the protein in an undenatured state."
It is suggested that the major function of soluble collagen in cosmetic preparations is as a highly efficient natural agent for the retention and enhancement of skin moisture.
The foregoing observations would appear to indicate that lower molecular weight collagens obtained by degradation or hydrolysis by native collagens may have some superficial activity in application to the skin.
Studies indicate that the stratum corneum is capable of absorbing and retaining only moisture and that it will not accept lipid or fatty substances. Further, when lipids are removed from the skin but water soluble substances are left in the skin, its ability to hold moisture is impaired. Accordingly, various preparations have been proposed which attempt to restore lipids to the surface of the skin accompanied by hygroscopic materials which attract and hold moisture. The preparations may include soluble collagens to improve surface texture of the skin as long as they remain applied to the surface thereof. Such formulations do not permit any significant penetration of the skin beyond the outermost layers thereof and therefore permit only transitory surface effects. Examples of such formulations are described in U.S. Pat. Nos. 3,548,056 and 4,454,159.
In the study of the structure of the stratum corneum, the dermatologists have used strips of plastic adhesive tape, such as, the well-known "Scotch" brand tape to remove successive layers of the stratum corneum with each piece of tape. This has been verified by staining of the cells removed by each layer followed by histological examination. For most individuals, the stratum corneum comprises aabout 12-18 layers of cells (sometimes considered as 10-20 layers). After this number of layers have been stripped from the same site of stratum corneum, there appears what is known as the "glistening layer" of the epidermis, which is so-called because at this layer tissue fluid starts to ooze out of the living cells.
It is believed that stripping off these layers of the stratum corneum followed by subsequent analyses of single or multiple layers gives a good indication of the degree of permeability of a material into the skin.